The backbone of DNA is based on a repeated pattern of a sugar group and a phosphate group. A phosphate group is attached to the sugar molecule in place of the -OH group on the 5' carbon.
The nitrogen bases attach in place of the -OH group on the 1' carbon atom in the sugar ring.
In the DNA chain, purines bond with pyrimidines.
A always pairs with T
C always pairs with G
DNA bases are held together by hydrogen bonds. There are two hydrogen bonds between A and T and three hydrogen bonds between C and G.
The Nitrogenous bases are to the centre of the helix, while the sugar and phosphate backbones are on the outside and exposed to the medium. The regular helical nature of the sugar-phosphate backbone does not allow space for two purines between the strands - they simply won't fit. Two pyrimidines will fit, but the bases will be too far away to form effective hydrogen bonds. A purine-pyrimidine pair fits perfectly.